The present invention relates generally to image sensing and, in particular, to dark current compensation.
Solid state imaging devices, such as, charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) imaging devices are used in different electronic devices, such as digital cameras, copiers and scanners. The imaging devices include a number of photodiodes arranged in a pixel matrix having vertical columns and horizontal rows. During operation, the photodiodes are exposed to a light source. For example, a shutter of a digital camera is opened to expose a photodiode array. The incident photons are converted to electrons and stored as a charge on the photodiodes. These charges represent the light image exposed to the photodiode matrix. The charges of the photodiodes are transferred to a processing unit for converting the charge to digital data. For example, an image captured by an array of photodiodes in a camera is processed and stored in memory for future viewing and/or printing.
A standard method of transferring an image captured by a CCD array is to sequentially read each pixel of a row, and then sequentially read each pixel of a subsequent row. This method is repeated until each pixel of the array has been read. Because the entire photodiode array is exposed to light simultaneously, and the photo diode charges are transferred serially, there can be a substantial time period between exposing the array and transferring a charge of the last photodiode pixel. Further, solidstate image devices are volatile. That is, photodiodes are susceptible to leakage current and therefore lose charge overtime. This charge loss is particularly noticeable when the array is dark, or no longer exposed to the light source. The photodiode leakage current during the dark period is referred to as a dark current. This dark current is a function of temperature, fabrication variables, and the length of time in which the photodiode is dark (exposure and read out time). A variable error, therefore, is induced during the time period between exposing the array and reading out the last photodiode. The dark current is always present in the imaging device. That is, dark current is present during both exposure (some times referred to as integration) and when there is not incident light.
Different approaches have been proposed for eliminating or compensating for dark currents. For example, U.S. Pat. No. 5,608,455 entitled xe2x80x9cInterline Transfer CCD Image Sensor with Reduced Dark Currentxe2x80x9d issued Mar. 4, 1997 describes a CCD image sensor which uses optical black areas located between an image pick up area and a horizontal transfer path. The CCD image sensor captures and stores a dark image of the active array. During the data transfer operation of an exposed image, the stored dark image of the active array is used to compensate for dark current errors. Additional approaches have been proposed which compensate for dark currents by manipulating transfer operations, or physical processing variations.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for accurate reduction of dark current errors in imaging devices.
In one embodiment, an imaging device comprises an imaging array comprising a plurality of photo sensitive pixels arranged in a plurality of rows and columns. The imaging array further comprises an active area, a first optically dark area comprising at least one row of pixels, and a second optically dark area located adjacent to the active area and comprising at least one column of pixels. A compensation circuit is coupled to the imaging array. The compensation circuit generates an initial dark current offset value from an output signal of the first optically dark area, and adjusts the initial dark current offset value using an output signal of the second optically dark area.
In another embodiment, an imaging device compensation circuit comprises an input node for receiving an output signal from an optically sensitive device, a buffer circuit coupled to the input node, a reference voltage circuit coupled to an input node of the buffer circuit via a first switch circuit, and an integrator circuit coupled to an output signal of the buffer circuit. The integrator circuit has an output node coupled to an input node of the buffer circuit via a second switch circuit.